Vacuum deposition processes, and in particular the fabrication of semiconductor-based integrated circuits for microelectronic or nanoelectronic devices (such as microprocessors, memory chips), generally involve plasma processes for cleaning, material deposition, patterning, material removal and doping. The chambers within which such plasma processes are performed are expected to present “clean”, chemically inert surfaces to minimise interference with the operations being performed and the devices being fabricated. The process chambers themselves, and components used within them such as electrodes, wafer support tables, gas inlets and outlets, are therefore made from a wide variety of materials ranging from stainless steel to ceramics such as quartz, alumina, and silicon carbide. Metallic components of such process chambers are generally made from stainless steel, with aluminium also being used, although there are material limitations and preferences depending on the exact processes to be conducted (e.g. the chemical environments to which the surfaces are exposed), and even depending on the materials being processed. Where aluminium is used, it is often anodised to present a more chemically inert, corrosion-resistant and plasma-erosion resistant surface. In some cases, where the protection offered by anodising is insufficient, metal components are even plasma sprayed with more erosion-resistant ceramics such as yttria or yttria-alumina cermets (see, for example, U.S. 2009/223450 and TW 557642).
Anodising of aluminium is generally used to enhance the corrosion and erosion-resistance of aluminium component surfaces. “Hard anodising” is particularly effective at this, and is the preferred means of enhancing the usable lifetime of components with aluminium surfaces exposed to plasma erosion or corrosion. Hard anodising is typically performed in a chilled bath of concentrated sulphuric acid (10-20 wt % H2SO4), with anodic potentials of between 30 and 90V applied to the aluminium surfaces at current densities of between 320 and 970 A/m2 (30 and 90 A/ft2). All forms of anodising grow aluminium oxide (Al2O3) by oxidation of the aluminium substrate. The aluminium oxide is amorphous and has a fine-scale columnar pore structure which is maintained by substrate dissolution during the anodising process. The scale of this porosity is minimised by the conditions of hard anodising, resulting in pore diameters of <100 nm. On rough surfaces, and particularly sharp convex radiuses (<100 μm), the coating is generally microstructurally defective, with opening cracks, because the simple, linear columnar growth process cannot compensate for the absence of sufficient substrate source material for an expanding volume of coating as it grows outwards on such a radius. Anodising is also limited to certain preferred substrate alloys because certain substrate alloying elements such as Cu, and precipitates such as silicates interfere with the formation of a uniform coating.
Numerous PEO technologies exist, including processes available from Magnesium Technologies Licensing, Ltd., Auckland, New Zealand, under the trade designation Anomag (WO 96/28591, and WO03/016596), processes available from Technology Applications Group, Grand Forks, N. Dak., under the trade designation Tagnite (U.S. Pat. No. 5,264,113) for surface treatment of magnesium, and processes available from Keronite Group, Ltd., Cambridge, United Kingdom, under the trade designation Keronite (WO99/31303, WO03/083181, U.S. Pat. No. 6,896,785) for protection of aluminium magnesium and titanium alloys. With regard to the coating of magnesium and its alloys, the afore-mentioned Anomag and Tagnite processes result only in amorphous magnesium phosphate coatings, whereas the specific process parameters of the Keronite process (as detailed for example in WO03/083181) generates crystalline MgO in the periclase form. The Keronite process has previously been used for the protection of disposable aluminium liners for protection of components in plasma process chambers (WO2007/092611), with electrolytes and process conditions being specifically formulated to give highly crystalline alumina with minimal incorporation of undesirable elements such as copper or potassium. If sufficient longevity can be achieved, it would be possible to dispense with the use of liners altogether, and to rely on a PEO coating alone, directly applied to plasma chamber components. Numerous PEO technologies exist, including Anomag (WO 96/28591, and WO 03/016596), Tagnite (U.S. Pat. No. 5,264,113) for surface treatment of magnesium, and Keronite (WO99/31303, WO03/083181, U.S. Pat. No. 6,896,785) for protection of aluminium magnesium and titanium alloys. With regard to the coating of magnesium and its alloys, the afore-mentioned Anomag and Tagnite processes result only in amorphous magnesium phosphate coatings, whereas the specific process parameters of the Keronite process (as detailed for example in WO03/083181) generates crystalline MgO in the periclase form. The Keronite process has previously been used for the protection of disposable aluminium liners for protection of components in plasma process chambers (WO2007/092611), with electrolytes and process conditions being specifically formulated to give highly crystalline alumina with minimal incorporation of undesirable elements such as copper or potassium. If sufficient longevity can be achieved, it would be possible to dispense with the use of liners altogether, and to rely on a PEO coating alone, directly applied to plasma chamber components.
The Keronite PEO process can generate the crystalline oxides of whatever substrate (or ‘parent’) metal is used. That is to say that on aluminium alloys, it will produce crystalline phases of Al2O3; on magnesium alloys, it will produce crystalline MgO; on titanium alloys, it will produce crystalline TiO2, and so on. Some of these crystalline oxide phases present good resistance to erosion by reactive plasmas (notably α-Al2O3, γ-Al2O3 and MgO periclase for resistance to fluorine based plasmas), but it has been found that the addition to the coating of other oxide phases such as Y2O3, Er2O3 or Dy2O3 would be preferable for further improved plasma resistance and for coating longevity in a reactive plasma environment. Clearly, some modification of the coating process is necessary to incorporate significant levels of any such “secondary” oxide phases into the coating—that is to say additional oxide phases based on elements that are not present at significant levels (>2 wt %) in the parent/substrate metal.
It is noted that most PEO processes will inherently form what could be regarded as “mixed oxides”: for example, on aluminium alloys, a mixture of amorphous alumina with α-Al2O3 and γ-Al2O3, would be formed, additionally incorporating alloying elements such as silicon and copper into the oxide; generally as impurities in the amorphous phase of alumina. With respect to the present invention, however, such oxides would simply be regarded as aluminium oxide with impurities, and would all be considered to be within the “primary oxide”, in this example, the oxide of the original aluminium alloy substrate. Similarly, on magnesium-aluminium alloys such as AZ91, a mixture of magnesium aluminium spinel, together with magnesium oxide might be formed by PEO processing, but for the purposes of the present invention, this inherent mixture of oxides formed simply by oxidation of the substrate would again be considered the “primary oxide”. Typical PEO process electrolytes often include compounds such as silicates or phosphates to promote growth and deposition of oxide, and these may become included in the coating as silicates or phosphates which might again be regarded as “mixed oxides”. Again, however, for the purpose of this invention, they are regarded as part of the primary PEO system (of standard electrolyte and substrate alloy): they exit purely to facilitate growth of an oxide on any given substrate and are not as deliberate and unusual “secondary” additions for the express purpose of creating a mixture of oxide phases with enhancing resistance to attack by particular chemical species (such as fluorine ions).
Suspensions of oxide powders within the PEO electrolyte are considered within WO 2007/092611 as a means of obtaining coating compositions other than the oxide of the parent metal (i.e. oxides other than alumina in the case of aluminium surfaces). However, that approach has proved to be of limited commercial use. Powders and colloids are typically poorly dispersed within the electrolyte, giving rise to non-uniform coatings. They also result in unacceptably high levels of erosion in the necessary recirculation systems for cooling and agitating the electrolyte. Embodiments of the present invention seek to address these problems by incorporating secondary additions (for example yttrium or other similar elements known to produce beneficial oxides) into the PEO coating by other means.